1. Field of the Invention
The invention relates to the use of graft polymers as gas hydrate inhibitors.
2. Description of the Background
It is known that gas hydrates, also termed clathrate hydrates, can form under certain conditions in media which comprise gas molecules, such as CO2 or hydrocarbons, e.g. C1-C4-alkanes, and water. These gas hydrates are composed of the gas molecules mentioned surrounded by a “cage” of water molecules. Gas hydrates of this type also occur when water is present in mineral oil mixtures or in natural gas mixtures and, for example during transportation, they can lead to blocking of the pipelines.
To prevent this, gas hydrate inhibitors are added to the mineral oil mixtures or natural gas mixtures.
WO 96/41784 and WO 96/41785 disclose gas hydrate inhibitors composed of a copolymer of N-methyl-N-vinylacetamide (VIMA).
U.S. Pat. No. 5,420,370, U.S. Pat. No. 5,432,292, WO 94/12 761 and WO 95/32 356 disclose polymeric additives for clathrate hydrate inhibition in liquid systems. These have a comonomer with a lactam ring in the polymer.
Polyvinylcaprolactam in particular, and also copolymers of polyvinylcaprolactam with, for example, vinylpyrrolidone, have a cloud point when dissolved in water, i.e. a certain temperature at which the polymer precipitates (inverse solubility). For pure polyvinylcaprolactam this is from about 30 to 35° C. A low cloud point such as this is sometimes disadvantageous for the gas hydrate inhibitor application, since the polymer can precipitate in the gas/oil/water phase which is to be conveyed if the temperature of this phase (i.e. including the water of this phase) is high, as is very likely to occur in practice. Use is therefore widely made of copolymers of vinylcaprolactam with, for example, vinylpyrrolidone, or else with other hydrophilic monomers which raise the cloud point, including, for example, ionic monomers which have ionic groups such as carboxyl, sulfonate or (quaternized) ammonium (WO 96/38492).
WO 96/38492 discloses gas hydrate inhibitors comprising a polymer which has a 3- to 15-membered ring bonded to the polymer via a particular linking unit (spacer).
Graft polymers per se are known from the prior art. For example, the German Patents DBP 1077430, 1081229, 1084917 and 1094457 describe processes for preparing various graft polymers, such as graft polymers of polyvinyl esters or modified polyvinyl alcohols. EP 285 038 discloses the use of graft polymers based on polyalkylene oxides as graying inhibitors. EP 44 995 discloses graft polymers of PVA.